This application is directed to lenses and more particularly to beveled eyewear lenses, methods for producing such lenses, eyewear incorporating such lenses and a tool for making such lenses.
Protective eyewear for sports activities is widely recommended by safety and sport organizations for individuals participating in any sport activity. Many different designs of protective prescription sport eyewear exist, with varying degrees of protection depending on the range of danger associated with the particular sport. Selection charts are published which illustrate the different types of protection available and set forth recommendations as to which of the protective device is most suitable for a given sport. An example of such a selection chart is available in the American National Standards, Z87.1 (2003) published by the American National Safety Institute.
With shatterproof polycarbonate lenses being the standard-of-care for all safety sport eyewear, frame failure has become the chief concern in protective safety eyewear/goggles. Frame failure is generally considered to consist of any detachment of the lens from the frame or any full thickness penetration of the lens.
Most sport related frames, by design, incorporate a thicker-gauge frame (plastic or metal), shock absorbing padding and a common sport V-bevel lens edge which is mounted to a frame which has a matching V-channel with a posterior retention lip which extends further inward (centrally) of the eyewear opening providing a stopping ledge for the lens when impacted.
A more recent method of edging/beveling lenses into sport related eyewear/goggles is a flat posterior beveled lens mounted/inserted into a frame with a matching flat posterior eyewire retention lip so that upon impact, the frontal impacting force is perpendicular to the eyewire flat lip, thereby increasing the frame and lens resistance to failure.
However, these methods of edging/beveling lenses for sport and protective oriented eyewear/goggles suffer certain common drawbacks.
For example, common to all sport and impact protective eyewear is a thicker-gauge frame combined with poor placement of shock absorbing padding, which causes the frame to sit higher and further from the wearer's eyes than the optimum (vertex distance) fitting distance of 13 to 15 mm giving a consistent visual perception change and sacrifice in comfort each time a wearer switches to a protective sport frame.
In addition, as with all channel eyewire retention systems (V-channel or flat lip), there is an inherent weakness in the lens to frame design due to a division of strength and function within the frame eyewire. Of the total eyewire thickness, the portion of frame eyewire anterior to the eyewire channel apex, the main purpose of which is to secure the lens into the frame and prevent the lens from falling out forward (away from the eye), has little bearing on impact force resistance. The portion of eyewire posterior of the channel apex, which carries the main burden of impact bearing forces, is relatively small, necessitating increased eyewire thickness.
A recent addition of sport safety bevel to the optical industry combines an anterior V-bevel with a posterior flat lip bevel, which is mounted/seated in a frame with a channeled eyewire. Although a flat lip bevel is new in use to channeled eyewire sports frames, the concept of a flat posterior lip bevel was commonly used in the past with a specific metal frame design named “Porsche,” in which the lens was mounted to the front frame surface and retained by prongs.
In common dress eyewear, “hide-a-bevel” is an industry standard for minimizing the negative effects of a lens edge and thickness. Hide-a-bevel is a lens edge beveling technique where the V-bevel protrudes a minimum amount necessary for keeping a lens mounted to a frame eyewire, with the remaining edge thickness being at an angle parallel to the mechanical center axis.
Another method used in conjunction with the hide-a-bevel for minimized edge effect is the one-third edge bevel rule of thumb, in which of the total lens edge thickness, the apex of the V-bevel is positioned one-third of the thickness from the lens front edge.
Used in conjunction with the one-third rule of thumb, the hide-a-bevel maximizes a lens edge cosmetic appearance by disguising a lens edge and bevel as much as possible.
Common dress frame eyewires also conform to the same methods for lens edge camouflage in that the channel is placed in proximity to the anterior third of the eyewire thickness. Occasionally, channel placement may vary according to thinness of eyewire.
Unfortunately, methods for enhancing lens cosmetic value do not apply for protective eyewear. As the name implies, it is important in dress eyewear to hide the bevel. It is the bevel that accentuates a lens edge thickness and image effect.
There is a negative cosmetic value associated with protective sport lip bevel designs in that, due to the centrally inward extension of the posterior lip of the lens, in conjunction with a minimum thickness standard of 2.0 millimeters polycarbonate lens, there is an exaggeration in unwanted tunneling effect of the lens. In non-sport and dress beveled lenses this effect is only apparent in higher-powered prescription lenses and is further concealed by use of the “hide-a-bevel” that reduces both V-bevel depth and the inward extension of the posterior portion of a lens residual lens thickness. In sport protective eyewear, the negative cosmetic effect is compounded by non-concealed frosted lens edges.
A further concern of protective sport eyewear is the larger lens size needed for increased peripheral vision, which, with the exception of corrective power, increases all frame and lens parameters including image effects. With an increased lens size, the lens becomes thicker and a rise in negative image effects including peripheral lens distortion occurs.
The frame facial wrap (face form) is used in sports eyewear for a more natural, snug fit to accommodate a larger lens and frame size. The frame wrap also minimizes peripheral distortion by attempting to maintain a constant distance of lens posterior surface from wearer's eye. There is an associated increase in the curvature of the lens anterior and posterior surfaces to accommodate a more curved frame and to minimize peripheral distortion. The steeper lens curves radically increase a lens thickness regardless of power and size. When combined with a larger lens size the thickness results are alarming. With the increased lens edge thickness, residual thickness becomes a greater issue in that, even with posterior shifting capabilities (where possible) a lens must not protrude from the posterior surface of the frame. The steeper curve also increases the lens edge and bevel angles, amplifying the unwanted negative image and cosmetic effects to both wearer and onlookers.
In view of the foregoing, it is clear current sports eyewear incorporate all the “don'ts” of cosmetic eyewear. Although impact protective, these designs exaggerate the same negative aspects that the optical industry has been trying to minimize.
Moreover, with an increase in consumer demand for large wrap-type corrective eyewear, the need for lens edge camouflaging is becoming a necessity in all eyewear designs. The hide-a-bevel method, due to its minimal bevel, and current retention lip designs are inadequate to accommodate large wrap-type eyewear for the reasons mentioned hereinabove. Channeled eyewear frame designs transmit and condense impacting forces to the frame eyewear retention lip. The frame designs accommodate a lens v-beveled edge, which creates a wedge effect that can result in eyewear splitting along the channel apex. The hide-a-bevel is not effective in addressing the problems associated with wrap-type eyewear because of insufficient bevel surface.
The use of heat for lens insertion for plastic frames can give rise to significant drawbacks. For example, human errors are possible because there are no instruments to accurately measure the amount of heat needed for different types of plastic in frames of varying thicknesses, compounded by varying edge thickness of a lens due to corrective power and shape. Heat insertion can cause a warping and/or twisting of the frame eyewire, changing the contact points of lens to frame. Stretching causes weak points along the eyewire and throws off the sizing ratio of frame to lens.
Corrective eyeglass wearers are just as concerned with the aesthetics of frame and lens appearance as they are with visual performance. Thus there is a need for a lens retention system and method of edge beveling for both protective and dress eyewear lenses which would minimize and disguise lens thickness while maintaining a high level of safety.
It would therefore be desirable to provide an ophthalmic lens with increased resistance to breakage and resistance to dislodgement from an eyewear frame wherein the outer circumferencing edge of the corrective lens has no corrective power, thereby avoiding negative image effects associated with corrective lens edges.
It also would be desirable to provide an outer circumferencing edge of a corrective lens permitting the lens impact bevel to be shifted anteriorly or posteriorly according to lens corrective strength and/or function, i.e., sports specific or common dress eyewear, for maximizing lens protective and cosmetic values without jeopardizing the integrity of frame impact protection.